Place duck carcasses on baking sheet lined with parchment paper; roast until brown (about 30 minutes); remove from oven; place in very large pot; add 3 gals. water and all remaining ingredients; bring to a boil; reduce heat to low; cook until stock is full of flavor (about 6 hours); remove from heat; strain through fine chinois into large container; discard solids; cool; cover with plastic wrap; reserve in refrigerator.

Using chilled die, grind duck legs, pork loin, bacon, and pork fat into cold bowl set in large bowl of ice; add mushroom mixture; season; stir in cilantro, scallions, and oyster sauce (sauté a little in a skillet to taste); shape into dime-sized meatballs; place on baking sheet lined with parchment paper; broil meatballs, turning, until brown on all asides but still medium-rare in center (about 5 minutes); remove from broiler; reserve (keep warm).